1. Field of the Invention
The present invention relates to an amplifier, and more particularly, to an output stage circuit and an operational amplifier thereof.
2. Description of the Related Art
In analog circuit, an output stage circuit plays a role of driving a load in the condition of less gain decreasing.
FIG. 1 illustrates a circuit diagram depicting a complementary metal-oxide semiconductor (CMOS) operational amplifier with class AB output stage 100 in the prior art. Referring to FIG. 1, the operational amplifier 10 includes a class AB output stage 100, two resistors R10 and R11, two capacitors C10 and C11, an amplifying circuit A10 and a bias circuit B10, wherein the bias circuit B10 includes two sub bias circuits B10-1 and B10-2. For the explanation of the operation of the operational amplifier 10, in this circuit diagram, and a plurality of nodes A, B, X and Y are marked out. The sub bias circuits B10-1 and B10-2 respectively control voltages of nodes A and B through the current sources IB1 and IB2 therein, so as to respectively control the bias voltages of nodes X and Y. Thus, the bias current Iout of the class AB output stage 100 is proportional to the currents of IB1 and IB2.
The bias method of the class AB output stage 100 has advantages of fast frequency response and simple circuit configuration. However, the folded-cascode FC configuration has to be used in this circuit and the transistors 101 and 102 in the class AB output stage respectively need resistors and capacitors (R10, C10) and (R11, C11). According to the description above, apparently the circuit at least has two limitation:
1. Two sets of resistor and capacitor (R10, C10) and (R11, C11) should be used in this circuit, and such utilization in an integrated circuit will occupy a lot of layout area, and resistance and capacitance thereof in the integrated circuit are inaccurate.
2. When a higher supply voltage, such as 5V, is applied to this circuit, a folded-cascode configuration FC must be used in the bias circuit B10. Thus, this circuit cannot be applied to simple two stage amplifier.